The present invention relates to light emitting diodes (“LEDs”) and in particular relates to high-brightness light emitting diodes formed from Group III nitride active structures on silicon carbide substrates.
In general, light emitting diodes represent one class of photonic devices that are commercially well-established for numerous applications. For the first few decades of the semiconductor era, however, such light emitting diode applications (although numerous and successful) were limited to relatively low-intensity applications such as indicator lights or small displays (e.g., handheld calculators). One reason for such limited application was based in the available semiconductor materials, most typically silicon (Si) and gallium arsenide (GaAs). Basically, and as well-understood in this art, a light emitting diode emits light based upon the recombination of electrons and holes under the influence of an applied bias across a p-n junction. Because the frequency (and thus the wavelength) of emitted light is directly related to the energy of the transition leading to the recombination (E=hv), the frequency of light emitted by a light emitting diode is based upon (and ultimately limited by) the material's bandgap.
In that regard, GaAs has a bandgap of 1.4 electron volts (eV). Accordingly, the highest-energy transitions that GaAs can produce are in the red, orange and yellow portions of the visible spectrum.
A number of commonly assigned patents and co-pending patent applications likewise discuss the theory and nature of light emitting diodes, including but not limited to U.S. Pat. Nos. 6,459,100; 6,373,077; 6,201,262; 6,187,606; 5,912,477; 5,416,342; and 5,838,706; and Published U.S. Applications Nos. 20020022290; 20020093020; and 20020123164. The contents of these are incorporated entirely herein by reference.
In order to further commercialize light emitting diode applications, however, colors other than red, orange and yellow must be available. Specifically blue and green light emitting diodes are required (along with red diodes) to create white light or full color displays. Because these colors represent higher-energy portions of the visible spectrum, they require larger transitions than the bandgaps of silicon or gallium arsenide can provide. Thus, in the last two decades, significant interest has been developed—and progress generated—in a wider bandgap materials such as silicon carbide (2.99 eV for the 6H polytype) and the Group III nitride's (e.g., 3.36 eV for gallium nitride). The Group III nitrides are particularly preferred because they are “direct” (and thus higher-efficiency) emitters.
In addition to providing blue, green, or white light (as well as emissions in the ultraviolet range), the Group III nitride light emitting diodes have the potential to provide replacement for long-standing illumination technologies such as incandescent and fluorescent lighting. In comparison to such mature technologies, light emitting diodes (“solid-state lighting”) are longer-lasting, physically more rugged, use less power, and are more efficient. Historically, however, LEDs have lacked brightness comparable to incandescent, fluorescent or vapor-discharge lights and thus these older technologies have continued to occupy the field. Only recently, have white LEDs (or LED-based white-emitting devices) begun to make inroads into commercial lighting applications, with most of these being in smaller applications such as flashlights and related items.
It will be understood of course that a single diode will not produce a white emission because white light is a combination of non-white frequencies. In some LED applications, however, a blue or UV-emitting LED can be used in conjunction with a phosphor to produce white light from a single diode source.
In commercial embodiments of light emitting diodes (e.g., the XBRIGHT™ diodes offered by the assignee herein; Cree, Inc.; Durham, N.C.) recent advances have included an inverted device design. U.S. Pat. No. 6,740,906 discusses aspects of this design as does U.S. Patent Application Publication No. 20020123164. The contents of both of these are incorporated entirely herein by reference. In this type of design, the Group III active layers are grown (typically epitaxially) on a silicon carbide substrate. Light emitting diodes of this type are then mounted with their epitaxial layers (“epilayers”) “down” rather than “up”; i.e., the silicon carbide portions forms the display face of the mounted device while the epitaxial layers are mounted to and face a circuit or wiring most typically referred to as a “lead frame” that provides the electrical connection to the diode. The silicon carbide-up orientation increases light extraction from the device as set forth in the '906 patent and the '164 publication.
Silicon carbide can also be conductively doped. This provides advantages in comparison to sapphire based Group III nitride diodes. Because sapphire is an insulator, two top wire bonds are typically required to mount a working sapphire-based diode. In comparison, silicon carbide devices can be “vertically” oriented; i.e., with ohmic contacts on opposite ends of the device. Such vertical orientation is directly analogous to diodes formed in other conductive semiconductor materials such as gallium arsenide (GaAs), and thus the same mounting orientations and techniques can be used.
Although these “inverted” devices have successfully provided significant practical and commercial improvements, their “epilayer-down” orientation requires different, and to some extent more sophisticated, mounting on lead frames. In particular, because the active portion (p-n junction, multiple quantum well, etc.) is positioned closely adjacent to the lead frame, avoiding short circuits or other undesired interactions between the active portion and lead frame becomes more difficult.
For example, conventional LEDs (including Group III nitride on SiC devices) are often mounted on the lead frame using conductive silver epoxy. Silver epoxy is a mixture of more than about 50 percent by weight of microscopic silver particles with epoxy resin precursors. When used to connect electronic devices to circuits (or circuit boards) the silver epoxy provides flexibility, relative ease of handling, conductivity and good heat transfer properties. Because silver epoxy is (purposely) applied as a viscous liquid, it can and will flow accordingly and, unless other steps are taken, will tend to surround lower portions of any diode mounted with it. As noted above, if the active portions are adjacent the lead frame, the flowing silver epoxy can contact the active portion and cause short circuiting or other undesired interactions.
As a result, many conventional light emitting diode mounting techniques are either too difficult, too unreliable or simply unavailable for inverted Group III nitride silicon carbide devices. Other specific techniques (e.g., application Ser. No. 10/840,463 filed May 5, 2004 and now published as Patent Application Publication No. 20050029533) should or must be incorporated to avoid these problems.
As another potential solution, the inverted device can be positioned on some sort of sub-structure, with the sub-structure being attached to the lead frame. Examples include U.S. Patent Application Publication No. 20030213965. The sub-structure is included to add sufficient thickness to remove the active portions farther from the lead frame and its silver epoxy or related materials. As set forth in No. 20030213965, however, soldering the device to a substructure can undesirably tilt the device with respect to the sub-structure and thereby exacerbate the short-circuiting problem.
As another problem, the extra thickness resulting from the presence of the substructure is also a disadvantage, because manufacturers of end-use products frequently demand smaller and smaller—including “thinner”—light emitting diodes. Such demand includes, for example, flat-panel displays on small devices such as cellular phones and personal digital assistants (“PDAs”).
Accordingly, it remains a continuing goal to increase the current capacity, light output (power) and light extraction (geometry) capabilities of inverted light emitting diodes while concurrently reducing their size and particularly reducing their thickness. It remains a similar goal to produce such diodes in designs that can be conveniently incorporated into lead frames, packages and circuits in a manner similar or identical to related diodes.